The drilling of printed circuit boards for providing accurately located and well defined openings for receiving the leads of electrical components is a well known art. Usually, the shank of the drills themselves may be of the order of 1/8 inch in diameter and are rotated at an extremely high speed by a motor driven holding spindle. Oftentimes a plurality of spindles are provided for holding a like plurality of drills so that simultaneous drilling of various holes in a number of circuit boards in side by side relationship can be carried out.
Normally, the drill shanks are held in the spindles by jeweler type collets with set screws to assure concentricity and reliability in the proper gripping of the drill shank. Such collets are not only expensive, particularly where several are required in the case of multiple drill systems, but in addition the changing of drills can become somewhat time consuming in that the set screws must be loosened and then retightened with the new drill shank in position.
In an effort to simplify the replacement of drills or the substitution of different drills in a spindle arrangement, it has been proposed to provide a centrifugal type holder for the drill shank wherein the drill structure can be very easily slipped into the spindle and then after starting the drill, centrifugal forces develop which result in a tight gripping of the drill. There is only one type of centrifugal holding arrangement of which we are aware and in this particular system, the drill is provided with a specially made cylinder of larger diameter than the drill shank in turn designed to fit within the spindle. Upper sections of this cylinder are free to move radially outwardly under centrifugal force such that they will frictionally engage the inside wall of the spindle bore receiving the cylinder and thus hold the drill to the spindle by centrifugal force under high speed rotation of the spindle.
While the foregoing centrifugal system permits fairly rapid interchanging of drills, each drill must be provided with the specific type of cylinder for cooperation with the spindle and again a relatively high cost is involved. Moreover, problems have developed with this type of centrifugal system in assuring concentricity of the drill as well as dynamic balancing of the system generating the centrifugal forces.